We have no way of knowing. You certainly can ask the police to become involved. Whether they will agree to become involved is anyone's guess.
Of course you do. Everyone has dozens of legal right.
Are these really the questions you intended to ask?
What were the "valuables" that you left in your relatives' care? How much are they worth? Why did you leave them with relatives? Where was the "locked box" located? Please describe the "locked box." When did you discover the items missing? When was the last time you knew they were located in the "locked box"? Have your relatives offered you any explanation as to what happened to them? Who had keys to the "locked box" and who had access to the place where it was located? Have you spoken with everyone who had keys and access?

